Deployment of communications systems can be a very costly process. Wireless bandwidth has become a very expensive commodity. In addition, system hardware is relatively expensive. One approach to deploying a communications system is to deploy cells utilizing the same number of carrier frequency's and bandwidth in each cell from the start of the system. Thus, individual cells may be deployed from the start of the system in a configuration which is intended to fully utilize the bandwidth intended to ultimately be used by the system.
For example, suppose an operator has a wideband spectrum. Traditionally, the operator has two options to deploy the communications systems. In the first option, the operator uses the entire wideband spectrum, e.g., in every sector of every cell, from the very beginning. The cost is that all the terminals have to be able to process the signals in the entire wideband channel, thereby increasing the terminal costs and battery power consumption. In the second option, the wideband spectrum is divided into multiple carriers. At the beginning, since the number of service subscribers tend to be relatively low, the operator deploys the communications system only in the first carrier, e.g., in every sector of every cell, from the very beginning, and leaves the other carriers unused. Later as the number of service subscribers increases and the first carrier becomes crowded, the operator then expands the system by deploying services in the second carrier. The procedure may repeat until all the carriers are utilized eventually. The problem of this approach is that when the first carrier is the only carrier used, there could be a significant amount of interference in the first carrier (thus limiting sector throughput), while the other carriers are completely idle. In addition, changes in the number of carriers and/or carrier frequencies in a cell over time can create problems for older WTs which were not intended to operate on the newly deployed carrier frequencies or do not know the availability of newly deployed carrier frequencies. This has, in many cases, made the initial deployment of wireless communications systems relatively expensive and/or often inefficient in terms of initial bandwidth utilization.
Various types of wireless communications systems are possible. Deployment and bandwidth under utilization problems tend to be associated with wireless communications systems regards of the particular communications method employed in the system.
Some communications systems use spread spectrum signals while other systems, e.g., narrow band systems, do not. In “Digital Communications” (3rd edition, page 695), J. Proakis provides the following definition of spread spectrum signals: “Spread spectrum signals used for the transmission of digital information are distinguished by the characteristic that their bandwidth W is much greater than the information rate R in bits/s. That is, the bandwidth expansion factor Be=W/R for a spread spectrum signal is much greater than unity.”
In a communication system, the information bits are generally transmitted as blocks of coded bits to combat errors in the communication channel, where each block is the minimum unit of channel coding. In the case where no channel coding is performed, each information bit can be considered a block.
Direct sequence code division multiple access (DS-CDMA) signal and hopped orthogonal frequency division multiplexing (OFDM) are two typical spread spectrum signals. In the DS-CDMA signal, a coded bit of any coded block is transmitted as a sequence of chips, where the time duration of a chip is much shorter than that of a bit. Suppose a bit is N times longer than a chip, then the bandwidth expansion factor, or spreading factor, is N.
Consider two methods of transmitting a block of coded bits in an OFDM system, as shown in FIG. 1 and FIG. 2. FIG. 1 is a drawing 100 plotting tone on vertical axis 102 vs time on horizontal axis 104. Each tone represents a segment of bandwidth in the frequency domain. The air link resource is represented by a grid 106 including 120 squares, each square representing one tone over one time interval. Grid 106 shows 10 distinct tones over 12 time intervals. In the first method illustrated by FIG. 1, the coded bits of a block are transmitted using the minimum number of tones. In FIG. 1, the same two tones 108, 110 are used all the time. A first block of coded bits 112 represented by 12 squares with diagonal line shading uses tones 108, 110 during a first time segment 116. A second block of coded bits 114 represented by 12 squares with dotted shading uses tones 108, 110 during a second time interval 118. In this case, the OFDM signal is not spread spectrum signal.
FIG. 2 is a drawing 200 plotting tone on vertical axis 202 vs time on horizontal axis 204. Each tone represents a segment of bandwidth in the frequency domain. The air link resource is represented by a grid 206 including 120 squares, each square representing one tone over one time interval. Grid 206 shows 10 distinct tones 208, 210, 212, 214, 216, 218, 220, 222, 224, 226 over 12 time intervals 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250. In the second method illustrated in FIG. 2, the coded bits are transmitted using hopped tones. A first block of coded bits 252 represented by 12 squares with diagonal line shading uses: tones (208, 216) during a first time interval 228, tones (212, 220) during a second time interval 230, tones (216, 224) during a third time interval 232, tones (212, 220) during a fourth time interval 234, tones (210, 218) during a fifth time interval 236, and tones (222, 226) during a sixth time interval 238. A second block of coded bits 254 represented by 12 squares with dotted shading uses: tones (214, 220) during a seventh time interval 240, tones (208, 224) during an eighth time interval 242, tones (216, 222) during a ninth time interval 244, tones (212, 218) during a tenth time interval 246, tones (210, 226) during an eleventh time interval 248, and tones (214, 222) during a twelfth time interval 250. In FIG. 2, at any given time instant, only two tones are used. However, for the entire coded block 252, 254, twelve tones are used. In this case, the OFDM signal is spread spectrum signal.
In view of the above discussion, it should be apparent that method and apparatus for implementing a phased deployment of a communication system would be beneficial. In addition, a system configuration that can achieve a high level of bandwidth utilization, even if constructed in phases which use different amounts of bandwidth and/or different numbers of carries before arriving at the final system configuration, would be both desirable and beneficial.